Design of a Model for SelfPropagating High Temperature Synthesis of TaC

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Design of a Model for Self-Propagating High
Temperature Synthesis of TaC

• Michelle McChesney
• Advisor Dr. Olivia Graeve
• San Jose State University
• Senior Design Project
• Spring 2002

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Objective

• To design, develop and test a numerical model
  for the self-propagating high temperature
  synthesis (SHS) of TaC.

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TaC SHS Process

• Characteristics
• High DHf of the products
• High temperature (1000-5000 K)
• High combustion velocity (1-100 mm/s)
• Large thermal gradients
• Advantages
• Energy efficient
• Fast
• High purity products
• Possibility of small particle size
• Possibility of forming non-equilibrium phases

O.A. Graeve, Dissertation, UC Davis
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Field-Assisted Combustion Synthesis
The reactants are resistively heated.
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Combustion Reaction
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Mathematical Description

• The process of combustion synthesis is described
  by a Fouriers relationship

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Heat in the System

• In the field-assisted SHS process the heat is a
  balance of four components
• conduction of heat through the green compact,
• chemical heat due to the exothermic reaction,
• electrical heat due to Joule heating, and
• dissipation of heat from the faces.

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Model Geometry
Physical Sample
Discrete Grid
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Program Layout
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Boundary Conditions
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Discretization of the Joule Term
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Linear Solution
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Voltage Gradient
10 Volts Applied
25 Volts Applied
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Extent of Conversion
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Voltage Effect on Temperature
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Conclusions

• Results show the same general trend as the
  experimental results.
• Numerical modeling is an effective technique for
  approximating experimental conditions, and
  running virtual experiments without producing
  hazardous waste.

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Future Work

• Rewrite program for higher resolution, i.e. the
  model needs more grid points.
• Include constitutive equations for particle
  growth.

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Acknowledgements

• Dr. Olivia Graeve
• Dr. Xiao-Yan Gong

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Thank you!
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Growth Kinetics
The large amounts of molten phase at the reaction
front allows for particle growth. Once the melt
is exhausted, there is no more appreciable growth
of the particles.
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Ta-C Phase Diagram
Ta 2629C
High adiabatic temperature, but slow kinetics.
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Mixing of reactants
Mixing of reactants takes place in a glass jar
with Al2O3 milling balls. The pressing of
the compact produces a green density of 50 for
Ta C.
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Sample Holder
The apparatus serves the function of
a resistively heated furnace, but the
heating element is the actual sample. The
ignition source is a tungsten coil which reaches
a temperature of 2800C.
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Effect of Electric Field on Tempe-ratureof TaC
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Effect of Electric Field on Wave Velocityof TaC
Note Reaction is much faster than the TiNi-TiC.